To the Antlion's Den

Earlier, you had the opportunity to explore the Antlion's Den. It's the cavern that's directly east of Damcyan. Whether you went there before or not, it's where you're going now. Travel all the way back through the Underground Waterway and Falls, and walk up to Damcyan. From there, head straight east. You'll cross a bridge, and immediately thereafter, the Antlion's Den entrance will be visible.

If there isn't a Full Moon, you may want to sleep in Damcyan's Inn—or use Tents on the World Map—until the desired phase appears. Without it, you won't be able to retrieve the "Sand Pearl" you've come for.

The first room you'll enter is chock full of treasures. If you want all of them, be sure to venture to the furthest southwest and northeast corners of the area. When you're done, the exit lies in the southeast. This will bring you into B2. Go straight down, and enter the first yellow door you spy. If you were here earlier, you'd have snagged the Lamia Harp, and the Treasure Chest will be vacant. Chances are you've bought a better instrument by now, but if you haven't, it's a nice item.

Exit the Lamia Harp room, and take the northward path to your left. When you reach the staircase, use it, and you'll return to B1. This little path leads to another yellow door, which conceals a Save Point, and three Treasure Chests. Clean it out, record your progress, and go back to B2. Make your way south-southwest, and you'll eventually reach the passage leading down into the Antlion's Nest.

Check any of the claws to receive your Sand Pearl. There's nothing else required of you in the Den, so it's time to head back to Kaipo.

When you visit Harley's bedside, select the Sand Pearl in your inventory, and all will be well. The story will progress, and you'll be off to Baron via ship. To reach the docks, walk out of town, and head straight east. Once your boating trip concludes, take a short trek to Baron Castle nearby. Edward will have a talk with Cecil in the Throne Room, and afterward, he'll prompt his group to return home.